Herpes simplex viruses cause considerable morbidity and mortality. They undergo a lytic, productive infection at the mucosal sites and spread into sensory ganglia, where they undergo a latent infection for the life ofthe host. Reactivation leads to recurrent infection and disease. Antiviral drugs have been defined that inhibit the lytic infection cycle, but there are no approaches that target the latent virus. We have defined the role of viral gene products such as LAT and ICPO in modulating the chromatin structure during lytic and latent infection, but further basic information is needed about these mechanisms for discovery of therapeutics that target HSV latent infection. In this application our specific aims are: a. To test hypotheses for possible mechanisms by which the HSV latency-associated transcript reduces lytic gene expression during acute infection and during latent infection of trigeminal ganglia: l_AT acts as a long noncoding RNA that recruits histone-modifying complexes to the viral genome, b. LAT leads to chromatin changes by serving as a precursor to an miRNA that reduces ICPO expression through studies of miRNA mutant viruses (with Coen lab), c. LAT regulatory DNA sequences act in a cis-acting manner to promote chromatin on the viral lytic genes, d. LAT leads to differential targeting of the viral genome through studies of genome targeting in cultured neurons with the Leib lab. 2. To define the mechanisms by which the HSV ICPO protein regulates chromatin structure during acute infection and latent infection of trigeminal ganglia. We have exciting unpublished results that ICPO mutant viruses have a different chromatin profile on their genome during latent infection. We will test the hypothesis that ICPO acts to alter the chromatin state by recruiting histone modification enzymes to viral and cellular genes. 3. To define Interactions between LAT and ICPO in regulating HSV chromatin. We will test the hypothesis that LAT forms duplex RNA with ICPO transcripts to recruit histone modifying enzymes by mutating the ICPO promoter or mutating the ICPO translational initiation site and by constructing LAT and ICPO double mutant viruses to determine their combinatorial effects on latent infection, and viral chromatin. RELEVANCE (See instructions): Herpes simplex viruses cause considerable genital, ocular and nervous system disease, and genital herpes increases the risk of HIV infection. There are drugs that target the active growth of herpes simplex virus but none that target the latent infection. This research will define basic mechanisms of herpes simplex virus latent infection and new targets for potential drugs to treat the latent infection of these viruses